KVM: x86: reserve bit 8 of non-leaf PDPEs and PML4Es in 64-bit mode on AMD

Bit 8 would be the "global" bit, which does not quite make sense for non-leaf
page table entries.  Intel ignores it; AMD ignores it in PDEs, but reserves it
in PDPEs and PML4Es.  The SVM test is relying on this behavior, so enforce it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>